cvs commit: src/sys/i386/include _types.h

Colin Percival cperciva at
Thu Mar 6 02:46:09 UTC 2008

Bruce Evans wrote:
> On Wed, 5 Mar 2008, Colin Percival wrote:
>> Bruce Evans wrote:
>>>   Change float_t and double_t to long double on i386.
>> Doesn't this have a rather severe performance impact on any code which
>> uses double_t?
> No.  As mentioned in the commit message, this has no performance effect
> except in cases where it avoids compiler bugs.  [...] if you use long double
> for memory variables then you get a severe performance impact and some
> space loss for the load instruction, since loading long doubles is
> much slower than loading doubles (about 4 times slower on Athlons).

Either I'm misunderstanding something, or you seem to be disagreeing with
yourself here... if I have the following code

double_t foo, bar, foobar;
foobar = foo + bar;

then prior to this change the processor loads and stores doubles, while
after this change the processor loads and stores long doubles, with the
associated performance penalty.

Colin Percival

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